(1) Field of the Invention
This invention relates to indicator devices and more particularly to a device for indicating the relative velocity of air passing over either side of an air foil.
(2) Description of the Prior Art
Adjusting the sails on a modern sailboat to obtain maximum forward hull speed for the boat involves taking into consideration a large number of interrelated variables, many of which are constantly subject to change as the sailboat is underway. The principal significance in sailing is the requirement that the trim of the sail be maintained at an optimum attack angle with respect to the wind.
Experienced sailors for many years have been using yarn or tape telltails located on both sides of their headsail to give an indication of sail trim. When sailing in good trim both telltails stream back parallel to the sail, following the smooth flow of air over both sides of the sail. When the jib or foresail is trimmed too tight or one is sailing too low the leeward telltail will flutter indicating that the sail is stalling. Conversely if one is too high the weather telltail will flutter indicating the sail is approaching a luff. In such a system of course it is sometimes difficult for the sailor to view both telltails and accurately judge their relative position to thus insure proper sail set.
One scheme developed to provide a more efficient means of determining the set of the sail and proper angle of attack is that disclosed in U.S. Pat. No. 3,654,807 wherein temperature sensitive devices are mounted on either side of the sail nearest the leading edge and by means of electronic instrumentation, flashing lamps or similar indicators, provide an indication of the relative laminar air flow over either side of the airfoil whereby corrections may be made to the angle of attack. Such a system obviously is expensive to both manufacture and maintain and its use may be more complicated than is desirable to many sailors.
Accordingly it is the object of the present invention to provide a new angle of attack indicating device that overcomes the objections outlined above to such simplified angle of attack indicators as telltails, yet is economic to manufacture and maintain and as simple in its mode of operation.